Gasification of carbonaceous materials to produce producer gas or synthesis gas or syngas comprising carbon monoxide and hydrogen is well known in the art. Typically, such a gasification process involves a partial oxidation or starved-air oxidation of carbonaceous material in which a sub-stoichiometric amount of oxygen is supplied to the gasification process to promote production of carbon monoxide as disclosed in PCT Patent Application No. WO 93/018341. Gaseous products produced by partial oxidation of carbonaceous materials are often treated in a high temperature heat treatment unit, e.g. for destruction of tar. See for example WO 2009/154788 that discloses a two stage gasifier in which carbonaceous material is fed to the first stage in which air, oxygen-enriched air or pure oxygen can be injected at controlled rate. The first stage temperature and oxygen input is controlled such that only partial oxidation of carbonaceous material occurs. The gaseous product from the first stage moves to the second stage (heat treatment unit). Pure oxygen is introduced into the second stage in order to accomplish cracking and partial oxidation of any tar contained in the gaseous stream from the first stage. Product syngas is removed from the second stage.
Syngas produced by gasification processes described in the art can be hot and needs cooling prior to downstream processing. Recovery and use of this heat content of hot syngas can be very important for process economics. Hot syngas comprising carbon monoxide generated in gasification apparatus, is cooled in a heat exchanger or waste heat boiler downstream of the gasification apparatus. See for example U.S. Pat. Nos. 6,435,139; 7,587,995 and 7,552,701.
At high temperature reducing environment carbon monoxide is a stable product. However when carbon monoxide is cooled, carbon monoxide may oxidize into carbon dioxide, and produce carbon (graphite) that precipitates as soot:2CO(g)→CO2(g)+C(s)
This reaction is generally known as Boudouard reaction and is believed to take place at or below about 760° C. Fouling of heat transfer surface caused by deposit of carbon can cause disruption in operation of a syngas cooler. It is, therefore, important to eliminate or reduce fouling of the syngas cooler.
Sulfur in carbonaceous matter transforms to H2S in reductive mode of operation, to SO2 in oxidative mode of operation. It is advantageous to make SO2 during start-up so that it can be scrubbed easily prior to disposal.
There is a need for method of operation of a process comprising gasifying carbonaceous material in a gasification apparatus to produce syngas comprising carbon monoxide (CO) and carbon dioxide (CO2) and cooling said syngas in a syngas cooler in a way that fouling or carbon deposit formation is reduced or eliminated.